Arc-image zone refining furnace

ABSTRACT

An arc-image zone refining furnace especially adapted for purification of ceramics. The furnace has an auxiliary heating means to effectively prevent a bar of the material to be purified from being cracked due to steep temperature gradients in the solid-liquid interface.

United States Patent Yoshihiro Kino; Yasuhiko Nakayama, both of Kadoma, Osaka, Japan Sept. 15, 1969 June 29, 1971 Matsushita Electric Industrial Limited Osaka, Japan inventors Appl. No. Filed Patented Assignee ARC-1MAGE ZONE REFINING FURNAC 4 Claims, 1 Drawing Fig.

. 13/1, 13/3 1nt.Cl HHOSb 11/00 Field of Search 219/349,

[56] References Cited UNITED STATES PATENTS 1,757,895 5/1930 AS611 219/390 1,930,836 10/1933 DAmico 219/390 FOREIGN PATENTS 112,869 6/1966 Netherlands 219/349 Primary Examiner-R. F. Staubly Attorney-McCarthy, Depaoli, OBrien & Price ABSTRACT: An arc-image zone refining furnace especially adapted for purification of ceramics. The furnace has an auxiliary heating means to effectively prevent a bar of the material to be purified from being cracked due to steep temperature gradients in the solid-liquid interface.

PATENTEDJUNZSIHYI 3.590.216

INVENTOR YOSHIHIRO KINO BY YASUHIKO NAKAYAMA ATTORNEY ARC-IMAGE ZONE REFINING FURNACE This invention relates to improvement in an arc-image zone refining furnace and more particularly to a new and improved arc-image furnace provided with an auxiliary heating means whereby cracking of a' bar of the material to be purified due to steep temperature gradients in the solid-liquid interface is effectively prevented.

Various techniques of purifying semiconductors, ceramics or the like have heretofore been developed. Among these techniques is a arc-image zone refining furnace which makes use of a carbon are or xenon are as the primary source of heat to locally melt these materials. In the furnace of this known type, the radiation emanating from the carbon are or xenon arc is focused into an intense image by one or more ellipsoidal mirrors at the focal point of one of the ellipsoidal mirrors. The material to be purified, when placed at the focal point of the ellipsoidal mirror, is heated to an extremely high temperature beyond its melting point and melts into liquid in a limited portion thereof. Thus, if the material in the form of a bar is moved relative to the focal point in such a manner that the limited portion (which will be hereinafter referred to as liquid zone") traverses the length of the bar, the material of the bar is purified, or more exactly, zone refined, and makes a highpurity single crystal bar.

The arc-image furnace as described above is often used to purify a bar of ceramics because of the extremely high temperatures available. It is well known that ceramics have the disadvantage that they are greatly affected by thermal shock. Thus, when the bar of ceramics is heated locally to a temperature beyond 2,000" C. in the furnace, steep temperature gradients occur between the radiation focused portion and the radiation unfocused portion, that is, across the solid-liquid interface. Such gradients produce many cracks in the bar as the liquid portion solidifies. These cracks are detrimental to the physical and electrical characteristics of the material. As a matter of fact, only one-tenth of the total volume of the thus formed single crystal can be utilized.

It is therefore an object of this invention to provide an improved arc-image furnace using an auxiliary heating means whereby cracking of a bar of the material to be purified due to steep temperature gradients in the solid-liquid interface is effectively prevented.

The object and advantages of this invention will be more apparent from the description of embodiment taken in conjunction with the accompanying drawing which shows schematically an arc-image furnace constructed in accordance with one embodiment of this invention.

In the drawing, the arc-image furnace indicated generally at 10 largely comprises, as customary, a radiation source 11 and an ellipsoidal mirror 12. As shown, a bar 13 of the material to be purified is positioned relative to the ellipsoidal mirror 12 so as to ensure that at least a portion of the bar 13 occupies the focal point 14 of the ellipsoidal mirror 12. The radiation emanating from the radiation source 11 is focused into an intense image by the ellipsoidal mirror 12 at the focal point 14 thereof, so that this particular portion of the bar exposed to the focused ration is heated to extremely high temperatures. Thus, only a portion of the bar 13 is fused to form a liquid zone 15. As is well known in the art, the bar 13 is slowly moved downward relative to the focal point 14 of the ellipsoidal mirror 12 so that a different portion of the bar 13 occupies the focal point 14. By so doing, the liquid zone 15 is moved to traverse the length of the bar 13.

According to this invention, an auxiliary heating means 16 is provided around the bar 13. The heating means 16 comprises a tubing 17 surrounding the bar 13 and made of quartz, for example, a resistance wire 18 wound around the quartz tubing 17, and a source of electric energy (not shown) connected to the resistance wire 18. The resistance wire 18 as used in this embodiment may be a platinum wire 3 meters long and 0.5 millimeters in diameter, for example. Here, it is im ortant to have the auxiliary heating means 14 located in a manner to surround the portion of the bar in which a single crystal has just been formed from the liquid. With this arrangement, the temperature gradients across the solid-liquid interface U can be reduced and the cracking of the crystal is effectively prevented. An electric power of about 300 watts may be sufficient to heat the bar.

As described above, it is possible to form a single crystal of ceramics having a very high melting point of about 2,000 C. without cracks produced in the bar by providing the additional heating means, according to this invention.

What we claim is:

1. An arc-image furnace adapted for zone refining a bar of a semiconductor, ceramics or the like, comprising a radiation source, means for focusing the radiation from the radiation source at one point, said point being occupied by at least a portion of the bar whereby said portion is heated to form a liquid zone, means for supporting and moving said bar relatively to said focusing means. and an auxiliary heating means for heating a portion of the bar which has been solidified into a single crystal to reduce the temperature gradients across the solid-liquid interface of the bar.

2. An arc-image furnace as set forth in claim 1, wherein said auxiliary heating means comprises a tubing which surrounds the bar, a resistance wire wound on the tubing and a source of electric energy connected to the resistance wire.

3. An arc-image furnace as set forth in claim 2, wherein said tubing is made of quartz.

4. An arc-image furnace as set forth in claim 2, wherein said resistance wire is made of platinum. 

1. An arc-image furnace adapted for zone refining a bar of a semiconductor, ceramics or the like, comprising a radiation source, means for focusing the radiation from the radiation source at one point, said point being occupied by at least a portion of the bar whereby said portion is heated to form a liquid zone, means for supporting and moving said bar relatively to said focusing means, and an auxiliary heating means for heating a portion of the bar which has been solidified into a single crystal to reduce the temperature gradients across the solid-liquid interface of the bar.
 2. An arc-image furnace as set forth in claim 1, wherein said auxiliary heating means comprises a tubing which surrounds the bar, a resistance wire wound on the tubing and a source of electric energy connected to the resistance wire.
 3. An arc-image furnace as set forth in claim 2, wherein said tubing is made of quartz.
 4. An arc-image furnace as set forth in claim 2, wherein said resistance wire is made of platinum. 